Conventional power supply units for video display devices, for example television sets, use a single converter for supplying the power necessary for heavy loads such as during watching TV and for light loads when the TV is turned off or in the remote control standby state by employing a self-excited power supply (hereafter referred to as "switching power supply"). This system, however, causes great loss in the remote control standby state because the energy continuously regenerates in the switching element and transformer. (Refer to Japanese Laid-open Patent H4-172090.)
One alternative system is to employ two power supplies: a main power supply and a power supply for a control means such as a microcomputer (hereafter referred to generically as the microcomputer). This system is described below with reference to drawings.
FIG. 6 illustrates an electric circuit diagram for the power supply unit of the prior art which employs two power supplies. First, the configuration of a circuit for the main power supply is explained. AC power 1 is connected to the input terminal of a main bridge rectifier 4 through a line filter 2 and a main relay switch 3. The output terminal of the main bridge rectifier 4 is connected to the parallel circuit of a series circuit of the primary winding 6 of a transformer 5 for the main converter and a field effect transistor (FET) 7, acting as a switching element, and a smoothing electrolytic capacitor 8. A secondary winding 9 of the transformer 5 for the main converter is connected to a DC output voltage terminal 12 for a deflecting and high-voltage circuit through a relay switch 10 and a rectifying diode 11. Secondary windings 13 and 14 are connected to DC output voltage terminals 17 and 18 for 15 V and 7 V signal processors (not illustrated) through respective rectifying circuits 15 and 16. A second winding 19 is connected to a DC output voltage terminal 21 for a 30 V audio circuit through a rectifying diode 20.
The output side of the rectifying diodes 11, 15, 16, and 20 are connected to smoothing electrolytic capacitors 22, 23, 24, and 25 respectively.
The DC output voltage terminal 17 for the 15 V signal processor is grounded through a current-limiting resistance 26, a Zener diode 27, a light emitting diode 29 in a photocoupler 28, a Zener diode 30, and a transistor 31. The current I.sub.BF which flows to the collector of a phototransistor 32 in a photocoupler 28 is the feedback current to a controller 33 for the main converter.
The output terminal of the controller 33 for the main converter is connected to the gate terminal of FET 7, and a resonant capacitor 34 is connected between the source and drain terminals of FET 7.
The voltage of a bias winding 35 of the transformer 5 for the main converter is integrated by a integrator 36 and is input to the controller 33 for the main converter.
The DC output voltage terminal 12 for the deflecting and high-voltage circuit is, through an error amplifier 37, connected to the contact point of a light emitting diode 29 in the photocoupler 28 and the Zener diode 30.
A .gradient. mark in the figure indicates the ground (GND) of the primary winding of the transformer 5 for the main converter. The same mark is used in other figures.
The DC output voltage terminals 17 and 18 are connected to signal processors (not illustrated) through a three-terminal regulator (not illustrated) which is turned on and off by the control signal from a microcomputer 41 described below.
The configuration of a circuit for the power supply for the microcomputer is described next.
The primary winding of a power transformer 38 for the microcomputer is connected to the output terminal of the line filter 2, and the secondary winding of the power transformer 38 for the microcomputer is connected to the input terminal of a bridge rectifier 39. The output terminal of the bridge rectifier 39 is connected to a regulator 40 which converts the power supply voltage to 5V for use by the microcomputer.
The output voltage of the regulator 40 is supplied to the microcomputer 41.
The microcomputer 41 controls ON and OFF of the main relay switch 3, relay switch 10, and transistor 31 as it receives the input signal from a remote control receiver 42 and a main key 43 on a television set.
A smoothing electrolytic capacitor 44 is connected to the output terminal of the bridge rectifier 39.
Next, the operation of the power supply unit of the prior art as configured above is described.
Firstly, the power supply for the microcomputer is discussed.
The power transformer 38 for the microcomputer is connected to AC power 1 through the line filter 2, and AC 12 V is output to its secondary winding.
The bridge rectifier 39 conducts full-wave rectification of the AC 12 V output and the smoothing electrolytic capacitor 44 smoothes it to generate the DC output.
Then DC 5 V generated by the regulator 40 is supplied to the microcomputer 41.
The microcomputer 41 receives and decodes the remote control signal from the remote control receiver 42 and the input signal from the main key 43 on the television set, and outputs ON and OFF control signals to the main relay switch 3 and relay switch 10. The microcomputer 41 also outputs other signals for performing a range of controls of the television set.
As long as the television set is plugged into AC power 1, the power transformer 38 for the microcomputer supplies DC voltage to the microcomputer 41, thereby allowing the microcomputer 41 to operate continuously.
Secondly, the main power supply is discussed.
When the power key is turned on using the remote control or the main key, the microcomputer 41 outputs the control signal for turning on the main relay switch 3.
When the main relay switch 3 turns on, the main bridge rectifier 4 conducts full-wave rectification of the voltage of AC power 1 and the smoothing electrolytic capacitor 8 smoothes it to generate DC voltage.
When the DC voltage reaches a certain level, the controller 33 for the main converter for switching power supply activates to generate voltage in the secondary windings 9, 13, 14, and 19 of the transformer 5 for the main converter.
Here, voltage is generated in the DC output voltage terminals 17, 18, and 21 at the secondary side but not in the DC output voltage terminal 12 because the relay switch 10 is turned off and the transistor 31 is turned on by the microcomputer 41. Voltage in DC output voltage terminals 17, 18, and 21 increases and levels off to a steady state when the voltage reaches a certain level.
Under the above conditions, no video image is displayed on the screen because the deflecting and high-voltage circuit (not illustrated) is connected to the DC output voltage terminal 12 which is not in operation. Since the audio output circuit (not illustrated) is also turned off, only the signal processors (not illustrated) connected to the DC output voltage terminal 17 and 18 are operational (hereafter referred to as the remote control standby mode).
When a certain time passes after the signal processors (not illustrated) are activated and output signals including horizontal and vertical drive pulses are stably output, the microcomputer 41 then outputs the control signal for turning on the relay switch 10 and turning off the transistor 31.
When the relay switch 10 turns on, the rectifying diode 11 rectifies the voltage generated in the secondary winding 9, and the smoothing electrolytic capacitor 22 smoothes it to supply DC voltage to the DC output voltage terminal 12.
The DC voltage gradually increases and levels off at a steady state when the voltage reaches a certain level. At this point the output voltage is stabilized at 140V, and the deflecting and high-voltage circuit and the audio output circuit operate so that the television is ready to be watched in a normal mode (hereafter referred to as the TV mode).
Thirdly, operation in the remote control standby mode is explained with reference to FIG. 7 illustrating operating waveforms. FIG. 7 shows the waveform at each part illustrated in FIG. 6 in the remote control standby mode.
In this mode, the relay switch 10 is turned off and the transistor 31 is turned on by the microcomputer 41 using its control signal.
FIG. 7(a) shows the drive waveform VOUT which is output from the controller 33 for the main converter to FET 7, and FIG. 7(b) shows the waveform ID of the current in FET 7. The current from the drain terminal to the source terminal is considered the forward direction.
FIG. 7(c) shows the voltage VDS between the drain and source of FET 7, FIG. 7(d) is the output voltage VS of the bias winding 35, and FIG. 7(e) is the waveform VDL of the voltage input to the main controller after integrated by the integrator 36.
When VOUT becomes H(HIGH) at the time t1, FET 7 turns on and the current ID starts to flow. Then, the primary current in the primary winding 6 induces magnetic flux in the transformer 5 for the main converter which accumulates energy. At the same time, the voltage is induced in the secondary windings 9, 13, 14, and 19 of the transformer 5 for the main converter.
However, since the relay switch 10 is turned off, the induced voltage is not applied to the rectifying diode 11. No secondary current flows because the polarity of the rectifying diodes 15, 16, and 20 are configured in a way to induce voltage to reverse bias.
Voltage is also induced in the bias winding 35, but it is configured to generate the voltage in the inverse phase against VOUT. Therefore, VS becomes a negative voltage. The waveform VDL which passes through the integrator 36, and input to the controller 33 for the main converter is clamped to 0V by the clipper/clamper circuit built in the controller 33 for the main converter. After the ON period set by the controller 33 for the main converter is completed, VOUT switches to L(LOW) at the time T2, and FET 7 turns off.
When FET 7 turns off, flyback voltage occurs in the primary winding 6 and secondary windings 9, 13, 14, and 19, which induces voltage in forward bias to the rectifying diodes 15, 16, and 20 at the secondary side.
Energy accumulated in the transformer 5 for the main converter is then discharged as secondary current through the secondary windings 13, 14, and 19. The current is smoothed by the smoothing electrolytic capacitors 23, 24, and 25, and the required voltage is output respectively from the DC output voltage terminals 17, 18 and 21.
On the other hand, the flyback voltage VS occurred in the bias winding 35 at the primary side becomes a waveform shown as VDL after it goes through the integrator 36, and is input to the controller 33 for the main converter. At the time T3, energy accumulated in the transformer 5 for the main converter has been completely discharged as secondary current, and the flyback voltage induced by the secondary windings 13, 14, and 19 reverses. This will cause reverse bias in the rectifying diodes 15, 16, and 20 at the secondary side, which will turn off the secondary current.
The flyback voltage produced in the primary winding 6 is reversed, and energy accumulated in the resonant capacitor 34 is discharged. This will start resonance with the inductance of the primary winding 6, and voltage VDS between the drain and source of FET 7 oscillates as shown in FIG. 7(c).
In general, with regard to switching power supply, the frequency for repeating the above ON and OFF cycle increases as the load becomes lighter, which will make it difficult to prevent electromagnetic interference.
Therefore, the controller 33 for the main converter has a function for setting the minimum OFF period to prevent the OFF period from being shorter than the set minimum OFF period.
In the remote control standby mode, energy in the transformer 5 is completely discharged by the time t3 which is within the set minimum OFF period. However, the OFF period continues until the minimum OFF period ends at the time t4.
After the minimum OFF period set to the time t4 is completed, the controller 33 for the main converter outputs H to VOUT. Operation after H is output to VOUT restarts from the time t1.
Fourthly, how the output voltage is controlled and stabilized in the remote control standby mode is explained. In this mode, the DC output voltage terminal 17 at the secondary side is stabilized at 15V. No current flows to the error amplifier 37 because the relay switch 10 is turned off and the voltage of the DC output voltage terminal 12 is 0V. The current in the photocoupler 28 flows from the DC output voltage terminal 17 through the current limiting resistance 26, Zener diode 27, light emitting diode 29 of photocoupler 28, Zener diode 30, and transistor 31.
If a 7.5 V Zener diode 27 and a 6.8 V Zener diode 30 are selected, and forward voltage in the light emitting diode 29 of the photocoupler 28 is considered around 0.7V, the total voltage is 15V. The output voltage of the DC voltage terminal 17 is stabilized as described below to this total 15V.
If the DC output voltage terminal 17 has a voltage higher than 15V, the current flowing to the light emitting diode 29 of the photocoupler 28 increases, thereby increasing the collector current in the phototransistor 32 at the primary side.
The collector current in the phototransistor 32 operates as a feedback current to the controller 33 for the main converter.
If the feedback current increases, the controller 33 for the main converter shortens the ON period of VOUT to reduce the drain current in FET 7 (the primary current in the transformer 5 for the main converter) so as to reduce energy accumulated in the transformer 5 for the main converter per unit time.
When energy accumulated in the transformer 5 for the main converter is reduced, the voltage in the DC output voltage terminal 17 at the secondary side falls due to the decrease in voltage in the secondary winding. If the output voltage is lower than 15V, the reverse operation takes place to increase the output voltage.
The signal processor (not illustrated) is a load connected to the DC output voltage terminal 17. Since the load does not vary significantly in the signal processor, the output voltage of the DC output voltage terminal 17 can be fully stabilized using the above stabilization control.
Fifthly, operation in the TV mode is explained with reference to the operating waveforms in FIG. 8.
FIG. 8 shows operating waveforms of each part in FIG. 6 in the TV mode. FIGS. 8(a) to (e) are waveforms at the same positions in FIG. 6 as indicated in FIG. 7.
Operation during the time t1 to t3 is the same as that in the remote control standby mode, but load is heavier in the TV mode than in the remote control standby mode. Therefore, as shown in FIG. 8(a), the ON period between the time t1 and t2, and the secondary current discharge period between the time t2 and t3 are longer than those in the remote control standby mode.
Drain current ID in FET 7, as shown in FIG. 8(b), is also larger in the TV mode than in the remote control standby mode.
When energy stored in the transformer 5 for the main converter is completely discharged as the secondary current at the time t3, the flyback voltage induced by the secondary windings 9, 13, 14, and 19 is reversed. Reverse bias then occurs in the rectifying diodes 11, 15, 16, and 20 at the secondary side and the secondary current will be turned off.
The flyback voltage generated in the primary winding 6 also reverses to discharge energy stored in the resonant capacitor 34, and resonance with the inductance of the primary winding 6 starts.
Then, as shown in (c), the voltage VDS between the drain and source of FET 7 starts to fall.
Here, the minimum OFF period set in the controller 33 for the main converter is shorter than the time t3. After the minimum OFF time is completed, FET 7 turns on at the time when VDL in FIG. 8(e) passes below the threshold VTH of the controller 33 for the main converter.
By setting the constant of the integrator 36 in a way that VDL passes below the threshold VTH at the time t4 when the resonance voltage of the voltage VDS between the drain and source is at a minimum, the controller 33 for the main converter turns on when detecting the reset of the transformer 5 for the main converter, and outputs H to VOUT at the time t4.
Operation after VOUT becomes H is same as that from the time t1.
Sixthly, how the voltage of the DC output voltage terminal 12 is stabilized in the TV mode is explained. In the TV mode, the transistor 31 is turned off, and the current flowing to the light emitting diode 29 of the photocoupler 28 is supplied from the DC output voltage terminal 17, and controlled by the error amplifier 37.
The error amplifier 37 has a built-in reference voltage so that the current flowing to the light emitting diode 29 in the photocoupler 28 decreases if the voltage of the DC output voltage terminal 12 is lower than the reference voltage.
Operation of the controller 33 for the main converter after this is the same as that in the remote control standby mode, and the voltage of the DC output voltage terminal 12 increases.
If the voltage of the DC output voltage terminal 12 exceeds the reference voltage in the error amplifier 37, the reverse operation takes place to reduce the voltage of the DC output voltage terminal 12. In this way, the voltage is maintained at the same level even when the voltage of the DC output voltage terminal 12 fluctuates.
If the power key on the remote control or the television set is pressed in the TV mode, the mode switches to the remote control standby mode following the reverse procedures as the above.
The power supply unit configured as above which employs two power supplies, however, still consumes a considerable amount of power in the remote control standby mode. In addition, it needs extra parts, such as a power transformer 38, bridge rectifier 39, and smoothing electrolytic capacitor 44 for producing a power supply voltage for the microcomputer 41. This will require a larger power transformer and other components, resulting in occupation of larger area on the printed circuit board and increased overall costs.
An object of the present invention is to solve the above disadvantages and provide a smaller and more inexpensive power supply unit, which can conserve energy when supplying power to a light load, such as in the remote control standby mode, for video display devices.